When a limited power source, such as a battery, is used for driving electrical devices, such as compressors, it sometimes occurs that a required power need will result in the power source draining relatively quickly. If the power source should also be used for other purposes, such as starting a vehicle at a later time, this is highly undesirable. Because the power level required in order to start a vehicle will in most cases be higher than the power level required to run a compressor for a refrigeration system, this may result in the power level of the power source dropping below the level required for starting the vehicle, but the power level will still be sufficient to run the compressor, and the compressor will therefore continue to run. As a consequence, it will not be possible to start the vehicle when this is desired at a later time.
One possible solution to this is to simply let the compressor run as normal until the power level of the power source is slightly above a lower power level limit required in order to start the vehicle. However, this may result in the compressor being stopped a relatively long time before the vehicle is restarted, and a consequence may be that insufficient refrigeration may be provided to a refrigerated volume during the time elapsing from stopping the compressor to starting the vehicle, and this may lead to a too high temperature in the refrigerated volume. This is highly undesirable.
It is therefore desirable to be able to control the power level of the power source in such a way that a sufficient power level is available for various purposes of the power source when this is desired.
One way of obtaining this is described in U.S. Pat. No. 6,469,487, disclosing a solar powered vapour compression refrigeration system. The refrigeration system comprises an insulated enclosure and a thermal reservoir. A photovoltaic panel converts sunlight into DC (direct current) electrical power. The DC electrical power drives a compressor that circulates refrigerant through a vapour compression refrigeration loop to extract heat from the insulated enclosure and includes a phase change material. As heat is extracted from the insulated enclosure, the phase change material is frozen, and thereafter is able to act as a heat sink to maintain the temperature of the insulated enclosure in the absence of sunlight.
Furthermore, a controller monitors the rate of change of a smoothed power voltage to determine if the compressor is operating below or above the available power maximum, and adjusts the compressor speed accordingly. In this manner, the compressor operation is adjusted to convert substantially all available solar power into stored thermal energy.
The method and apparatus described in U.S. Pat. No. 6,469,487 does not, however, ensure that a sufficient power level will be available from the power source for other purposes than refrigeration at times where the power source is limited, i.e. no sunlight is available.